Characterization of the peptidoglycan of vancomycin-susceptible Enterococcus faecium

Vancomycin and other antibacterial glycopeptide analogues target the cell wall and affect the enzymatic processes involved with cell-wall biosynthesis. Understanding the structure and organization of the peptidoglycan is the first step in establishing the mode of action of these glycopeptides. We have used solid-state NMR to determine the relative concentrations of stem-links (64%), bridge-links (61%), and cross-links (49%) in the cell walls of vancomycin-susceptible Enterococcus faecium (ATTC 49624). Furthermore, we have determined that in vivo only 7% of the peptidoglycan stems terminate in D-Ala-D-Ala, the well-known vancomycin-binding site. Presumably, D-Ala-D-Ala is cleaved from uncross-linked stems in mature peptidoglycan by an active carboxypeptidase. We believe that most of the few pentapeptide stems ending in D-Ala-D-Ala occur in the template and nascent peptidoglycan strands that are crucial for cell-wall biosynthesis.